Future Work

Based on a full understanding of the production of traditional high performance fibres, the electrospinning process and recently reported mechanical properties of electrospun nanofibres, this thesis presents several routes to electrospun fibres with highly oriented molecular structures. Although the electrospun BPO nanofibre has been proven to be among the best nanofibres with regards to mechanical properties, there is still a big gap between the mechanical properties achieved here and those of conventional high performance polyimide fibres such as BPDA/PPD/PMR whose tensile strength can be as high as 5.1 GPa with Young‟s moduli up to 282 GPa [17, 18]. The significantly lower mechanical properties of our electrospun co-polyimide fibres can again be mainly ascribed to the relatively poor molecular chain orientation in these fibres as reflected by a Herman‟s orientation factor of BPO nanofibres of about 0.8, compared to orientation factors in commercial PI fibres approaching 1. Thus, a potential way to further improve mechanical properties is through improved molecular orientation in these electrospun fibres.

Although post-drawing of single nanofibres is hardly technologically feasible, a certain degree of stretching of nanofibre bundles or UD mats could be used to further improve molecular orientation or even increase crystallinity [19]. Some preliminary data is shown in Figure 7.2. Here, a certain degree of stretching (3%, 5%, 7%, 10%) on electrospun BPO polyamic acid nanofibre UD mats was applied in a universal testing machine enclosed in an oven at different temperatures ranging from room temperature to 120 °С (i.e. below the imidization temperature). Young‟s modulus of

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all samples was significantly enhanced to about twice its initial value from 2.1 GPa to 3.9 GPa. It should be noted that PAA nanofibre can only be stretched to about 7% at 100 °С and 3% at 120 °С, respectively. Further stretching would lead to fibre breakage. However, the modulus of all stretched UD nanofibre mats only increased by about 10% to 20% as shown in Figure 7.3, which is well below our expectations. A possible reason for this could be that stretching barely improved the molecular orientation but only improved the nanofibre orientation in the mat.

Figure 7.2. The Young’s modulus of electrospun BPO polyamic acid nanofibres

after stretching at different extensions and temperatures.

An alternative approach worthy of trying is to apply tension to the fibre during the imidization process. For this, PAA fibres were stretched at room temperature or 50 °С to about 10% extension, with subsequently a certain degree of pre-tension

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being applied to the stretched nanofibre mats during the imidization process. It is expected that this could further improve the orientation during the molecular self- ordering process from flexible to rigid chain structure.

Figure 7.3. The corresponding Young’s modulus of stretched electrospun BPO

nanofibres after imidization.

As described in Section 4.3.1, after UV-curing, the blend nanofibres tended to fuse together, as the liquid crystals were prone to leach out from the blend fibre. Bi- component electrospinning has been successfully applied to produce core-shell fibres with a polymer [20] or liquid crystal [21-23] core. Hence, some preliminary bi- component electrospinning studies were performed to evaluate to potential of this technique while at the same time solving the leaching problem as the RMs would be contained in a thermoplastic shell.

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Figure 7.4. Schematic illustration of the experimental set up for bi-component

electrospinning. The insets show the diameter of co-axial spinneret and a core-shell structure of nanofibres.

7 wt% PMMA in formic acid and acetic acid (1/1, w/w) and 21 wt% of RM257 dissolved in chloroform and p-xylene (1/1, w/w) were used as outer and inner solutions, respectively. Feeding rate was about 1 ml/h for both solutions, voltage around 20 kV and distance of 20 cm were adjusted to obtain good quality nanofibres.

Figure 7.5. SEM micrograph of core-shell nanofibres with RM257 as core and

PMMA as shell. ground High voltage - + + + P MMA L iq u id cry sta ls P M M A

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Figure 7.6. TEM micrographs of electrospun nanofibres showing clear contrast

between core (RM257) and shell structures (PMMA); (a) before UV-curing and (b) after UV-curing.

Core-shell nanofibres composed of a liquid crystal core (RM257) and a polymer shell (PMMA) were created using an electrospinning set-up incorporating a bi- component spinneret as shown in Figure 7.4. Figure 7.5 clearly displays the core- shell structures of these electrospun nanofibres after photo-polymerization by means of SEM and Figure 7.6 presents two TEM graphs with clear contrast of a core-shell morphology before and after UV-curing. No nanofibre fusion was observed because the polymer shell now inhibits flow of the liquid crystals.

However, the numerous process parameters including solvent selection (miscible or non-miscible) [24] for both core and shell, evaporation rate mismatch between both solvent systems (which can result in re-dissolving of core or shell) and the flow rate adjustment of both solutions caused great complexity in the bi-component electrospinning process for RM257 and PMMA. Nevertheless, the capability of embedding liquid crystals inside a bi-component nanofibre could prove to be a

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potential way to create neat liquid crystal fibres with interesting functional and optical properties after removal of the polymer sheath.

7.3 References

1. Carrizales, C.; Pelfrey, S.; Rincon, R.; Eubanks, T.M.; Kuang, A.; McClure, M.J.; Bowlin, G.L.; Macossay, J. Thermal and mechanical properties of electrospun PMMA, PVC, Nylon 6, and Nylon 6, 6. Polymers for Advanced

Technologies 2008, 19, 124-130.

2. Bazbouz, M.B.; Stylios, G.K. The tensile properties of electrospun nylon 6 single nanofibers. Journal of Polymer Science Part B: Polymer Physics 2010,

48, 1719-1731.

3. Zussman, E.; Burman, M.; Yarin, A.; Khalfin, R.; Cohen, Y. Tensile deformation of electrospun nylon‐6, 6 nanofibers. Journal of Polymer Science

Part B: Polymer Physics 2006, 44, 1482-1489.

4. Sanatgar, R.H.; Borhani, S.; Ravandi, S.A.H.; Gharehaghaji, A.A. The influence of solvent type and polymer concentration on the physical properties of solid state polymerized PA66 nanofiber yarn. Journal of Applied Polymer

Science 2012, 126, 1112-1120.

5. Hang, F.; Lu, D.; Bailey, R.J.; Jimenez-Palomar, I.; Stachewicz, U.; Cortes- Ballesteros, B.; Davies, M.; Zech, M.; Bödefeld, C.; Barber, A.H. In situ tensile testing of nanofibers by combining atomic force microscopy and scanning electron microscopy. Nanotechnology 2011, 22, 365708.

6. Stachewicz, U.; Peker, I.; Tu, W.; Barber, A.H. Stress delocalization in crack tolerant electrospun nanofiber networks. ACS Applied Materials & Interfaces 2011, 3, 1991-1996.

187

7. Veleirinho, B.; Rei, M.F.; Lopes‐DA‐Silva, J. Solvent and concentration effects on the properties of electrospun poly (ethylene terephthalate) nanofiber mats. Journal of Polymer Science Part B: Polymer Physics 2008, 46, 460-471. 8. Wu, S.Z.; Yang, X.P.; Zhang, F.; Hou, X.X. Stretching-induced orientation for

improving the mechanical properties of electrospun polyacrylonitrile nanofiber sheet. Advanced Materials Research 2008, 47, 1169-1172.

9. Pai, C.-L.; Boyce, M.C.; Rutledge, G.C. Mechanical properties of individual electrospun PA 6(3)T fibers and their variation with fiber diameter. Polymer 2011, 52, 2295-2301.

10. Naraghi, M.; Arshad, S.; Chasiotis, I. Molecular orientation and mechanical property size effects in electrospun polyacrylonitrile nanofibers. Polymer 2011,

52, 1612-1618.

11. Papkov, D.; Zou, Y.; Andalib, M.N.; Goponenko, A.; Cheng, S.Z.; Dzenis, Y.A. Simultaneously strong and tough ultrafine continuous nanofibers. ACS

Nano 2013, 7, 3324-3331.

12. Chen, F.; Peng, X.; Li, T.; Chen, S.; Wu, X.-F.; Reneker, D.H.; Hou, H. Mechanical characterization of single high-strength electrospun polyimide nanofibres. Journal of Physics D: Applied Physics 2008, 41, 025308.

13. Baji, A.; Mai, Y.-W.; Wong, S.-C.; Abtahi, M.; Du, X. Mechanical behavior of self-assembled carbon nanotube reinforced nylon 6, 6 fibers. Composites

Science and Technology 2010, 70, 1401-1409.

14. Zussman, E.; Chen, X.; Ding, W.; Calabri, L.; Dikin, D.; Quintana, J.; Ruoff, R. Mechanical and structural characterization of electrospun PAN-derived carbon nanofibers. Carbon 2005, 43, 2175-2185.

15. Zhou, Z.; Lai, C.; Zhang, L.; Qian, Y.; Hou, H.; Reneker, D.H.; Fong, H. Development of carbon nanofibers from aligned electrospun polyacrylonitrile

188

nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties. Polymer 2009, 50, 2999-3006.

16. Arshad, S.N.; Naraghi, M.; Chasiotis, I. Strong carbon nanofibers from electrospun polyacrylonitrile. Carbon 2011, 49, 1710-1719.

17. Kaneda, T.; Katsura, T.; Nakagawa, K.; Makino, H.; Horio, M. High- strengthhigh-modulus polyimide fibers I. One-step synthesis of spinnable polyimides. Journal of Applied Polymer Science 1986, 32, 3133-3149.

18. Kaneda, T.; Katsura, T.; Nakagawa, K.; Makino, H.; Horio, M. High-strength- high-modulus polyimide fibers II. Spinning and properties of fibers. Journal of

Applied Polymer Science 1986, 32, 3151-3176.

19. Sikkema, D.J. Design, synthesis and properties of a novel rigid rod polymer, PIPD or M5: high modulus and tenacity fibres with substantial compressive strength. Polymer 1998, 39, 5981-5986.

20. Sun, Z.; Zussman, E.; Yarin, A.L.; Wendorff, J.H.; Greiner, A. Compound core-shell polymer nanofibers by co-electrospinning. Advanced Materials 2003,

15, 1929-1932.

21. Lagerwall, J.P.; McCann, J.T.; Formo, E.; Scalia, G.; Xia, Y. Coaxial electrospinning of microfibres with liquid crystal in the core. Chemical

Communications 2008, 5420-5422.

22. Buyuktanir, E.A.; Frey, M.W.; West, J.L. Self-assembled, optically responsive nematic liquid crystal/polymer core-shell fibers: Formation and characterization. Polymer 2010, 51, 4823-4830.

23. Wu, Y.; An, Q.; Yin, J.; Hua, T.; Xie, H.; Li, G.; Tang, H. Liquid crystal fibers produced by using electrospinning technique. Colloid and Polymer Science 2008, 286, 897-905.

189

24. Longson, T.J.; Bhowmick, R.; Gu, C.; Cruden, B.A. Core–shell interactions in coaxial electrospinning and impact on electrospun multiwall carbon nanotube core, poly (methyl methacrylate) shell fibers. The Journal of Physical